This invention relates to supramolecular polymers comprising quadruple hydrogen bonding units that are capable of forming at least four H-bridges with each other in a row leading to physical interactions between different polymer chains. The physical interactions originate from multiple hydrogen bonding interactions (supramolecular interactions) between self-complementary units comprising at least four hydrogen bonds in a row. Units capable of forming at least four hydrogen bonds in a row, i.e. quadruple hydrogen bonding units, are in this patent application abbreviated as 4H-units. Sijbesma et al. (U.S. Pat. No. 6,320,018; Science 278, 1601, 1997; incorporated by reference herein) discloses 4H-units that are based on 2-ureido-4-pyrimidones. These 2-ureido-4-pyrimidones in their turn are derived from isocytosines.
Telechelic polymers or trifunctional polymers have been modified with 4H-units (Folmer, B. J. B. et al., Adv. Mater. 2000, Vol. 12, 874; Hirschberg et al., Macromolecules 1999, Vol. 32, 2696; Lange, R. F. M. et al, J. Polym. Sci. Part A, 1999, 37, 3657-3670). However, these polymers are obtained by reaction in chloroform or toluene, both toxic organic solvents, and need prolonged reaction times of several hours in order to reach completion.
Polymers with 4H-units grafted on the main chain have been obtained by copolymerizing an olefin bearing a 4H-unit with a common olefin (Coates, G. W. et al., Angew. Chem. Int. Ed., 2001, Vol. 40, 2153). However, complex chemistry has to be used to prepare the monomer. Additionally, the monomer must be polymerized by a Ziegler-Natta catalyst which is known as being sensitive for oxygen and moisture. Moreover, the reaction has to be performed in dilute toluene solution, thereby worsening the reaction economy because of the need of removal of large amounts of organic solvent. Hence, such a synthesis process is commercially less attractive.
WO 02/46260 discloses polyurethane based polymers with 4H-units as end-cappers that can be used as hot melt adhesive. Example 4 in this patent discloses the preparation of supramolecular polyurethane polymers which are obtained by the bulk reaction of 6-methyl-isocytosine with 4,4′-methylene bis(phenyl isocyanate) (MDI) end-capped polyesters in the melt at 180° C., said reaction being performed in a Brabender mixer with a residence time of not more than 3 minutes. In this process it is preferred that the 6-methyl-isocytosine is finely milled to a particular particle size to facilitate rapid and efficient conversion.
JP A2 2004250623, incorporated by reference, discloses polyester diols derived from poly(butanediol terephthalate) or polylactide that are reacted in the melt with an isocyanato functional 4H-unit obtained by the reaction of diisocyanatohexane with 6-methyl-isocytosine. The reaction proceeds by kneading at 150° C. to 300° C., preferably at 160° C. to 250° C. and more preferably at 180° C. to 230° C. JP 2004250623 further discloses that it is desirable to perform the reaction above the melting point of the polymer. However, in order to control decomposition of the reactants and final products, the reaction is desirably performed at a temperature as low as possible, provided that the reactants are prevented to solidify as much as possible during the reaction. According to the examples, the reaction requires temperatures of 200° C. or higher and an excess of the isocyanato functional 4H-unit. Comparable functionalisation of poly(butanediol terephthalate) and poly(butanediol isophthalate) with this isocyanato functional 4H-unit at temperatures above 180° C. are also disclosed by Yamauchi et al. (Macromolecules 2004, Vol. 37, 3519). In both cases the excess of the 4H-unit in the synthesis has been removed using organic solvents (Soxhlet-extraction with methanol or precipitation from HFIP), thereby re-introducing the need of (toxic) organic solvents into the process. Moreover, the occurrence of side reactions with the isocyanate functional compound, like allophonate, biuret or isocyanurate formation, is eminent at the temperatures applied (High Polymers Vol. XVI, Polyurethanes: chemistry and technology, Part 1, Ed.: J. B. Saunders and K. C. Frisch; J. Wiley & Sons 1962).
US 2004/0087755 and US 2007/0149751, both incorporated by reference, disclose a process for the manufacture of a supramolecular polymer wherein a mixture of a polyol, a chain extender, a diisocyanate, an amino-functional organic powder and optionally a catalyst are heated to a temperature of about 100° to about 250° C., preferably in a twin-screw extruder. The amino-functional organic powder has an average particle size of less than about 100 μm and is preferably selected from particular pyrimidine, isocytosine, pyridine, pyrimidone, uracil and pyridine compounds. However, Examples 7 and 8 disclose that the minimum temperature for manufacturing is 150° C. or higher, because of the high melting point of the used 6-methyl isocytosine.
Clearly, there is a need for a general production process for supramolecular polymers containing 4H-units that does not require organic solvents because of toxicological, ecological and economical reasons. Moreover, there is a need for a process in the melt that can be performed at temperatures below 150° C. in order to prevent thermal degradation or the occurrence of side reactions and to reduce the amount of energy necessary in the production process. There is also a need for a broad range of monomers comprising 4H-units or precursors of these 4H-units that can be used comfortably in reactive processing due to their low melting point and easy processing.
The present invention discloses novel 4H-unit building blocks that have melting points below 230° C. and building blocks, which are usually isocytosines, that are precursors of 4H-units, having melting points below 295° C. It was unexpectedly found that small changes on the isocytosine ring or on the ureido-position result in lowering of the melting points and, more importantly, in a large lowering of the temperature required to perform reactive processing. This makes it possible to prepare new supramolecular polymers using reactive processing techniques at temperatures below 150° C. without the occurrence of isocyanate side-reactions, resulting in supramolecular polymers containing one or more 4H-units with excellent mechanical properties.